The novel IR devices and fabrication processes to be described are related to the types of IR detector arrays recorded in (1) U.S. Pat. No. 4,080,532, Hopper, March 1978; (2) U.S. Pat. No. 4,745,278, Hanson, May 1988; (3) U.S. Pat. No. 4,792,681, Hanson, December 1988; and (4) "LOW-COST UNCOOLED FOCAL PLANE ARRAY TECHNOLOGY", by Hanson, Beratan, Owen and Sweetser, presented Aug. 17, 1993 at the IRIS Detector Specialty Review.
The physical requirements of uncooled arrays and a brief description of current fabrication processes will be presented to aid in the understanding of the improvements realized by the novel methods to be described.
An area imager may contain several hundred to tens of thousand individual picture elements (pixels). Each of these pixels consists of a capacitor (or resistor or another type of electronic element) that has a heat (IR intensity) sensitivity. Making use of the fact that the charge stored by a capacitor is proportional to the product of its terminal voltage and its capacitance, electronic circuitry can be attached to the two terminals of the capacitor based pixel to measure the intensity of the IR impinging on a specific pixel. Obstructions in the imaging field are removed and the electronic connections to these capacitors are simplified if one of the capacitor terminals is made common to all. Hundreds to tens of thousands of connections must still be made between the other isolated terminals of the capacitors and the electronic sensing circuitry. In addition, the pixel capacitors should be thermally isolated from each other while having one terminal connected to all the other common terminals.
The common connection to one side of the pixel capacitors consists of a front side thin film referred to as the optical coating. This may be a composite of a plurality of thin films which have been optimized with regard to IR absorbency, IR transparency, electrical conductivity, thermal resistivity, etc. The thicker heat sensitive dielectric substrate in this case can be barium-strontium-titanate (BST) which is a ceramic perovskite material.
To leave the front side optical coating thin film electrically conducting while isolating the pixels thermally, one may etch deep trenches around the pixel capacitors in the BST substrate. The optical coating side of the device will be referred to as the front side.